Field of the Invention
This invention relates to a support of photographic paper comprising a paper sheet coated with a polyolefin resin on both surfaces thereof. More particularly, the invention relates to a polyolefin-coated support of photographic paper whose back surface is provided with a writing-retainable layer comprising a crystalline silica and a film-forming organic polymer, whereby penciled characters, marks, ink, etc., can be firmly retained on the surface.
There has been previously employed as the photographic paper a baryte paper. Recently, however, a water-proof photographic paper comprising a paper sheet coated with a polyolefin resin on both surfaces thereof has been developed and employed to cope with requirement for a rapid development process. At the present time, most of the baryta papers have been replaced with the water-proof photographic papers. Generally, the water-proof photographic paper is coated with a photographic emulsion layer on one surface of the support (ordinarily, on a polyolefin layer containing a inorganic pigment such as titanium dioxide). In the present specification, the surface to be coated with the emulsion layer is referred to as "front surface", while a surface not to be coated with the emulsion layer and to reside opposite to the front surface is referred to as "back surface".
For a variety of purposes, it is desired that the back surface is able to receive and retain thereon characters, letter, marks, etc. of colorants such as oily and aqueous inks of ball-point pen or fountain pen, carbon colorant of pencil, and the like. Also desired is that the back surface can receive print.
However, since the water-proof photographic paper is coated with a polyolefin (generally, polyethylene), the surface is hydrophobic and not absorptive. Accordingly, it is difficult to write characters and the like on the surface with a pencil, fountain-pen, etc. Otherwise, scratches are liable to be given onto the surface when these writing means are applied thereonto. Further, the written characters rub off very easily, and this is also disadvantageous.
For providing the polyolefin coating surface with capacity for receiving the written or printed characters, etc. (referred to herein as "writing property"), a surface-roughening process by sand-blasting, embossing, etching with an acid or the like, etc., has been employed. However, the so processed polyolefin surface still does not have satisfactory writing property.
A variety of improvements have been proposed for eliminating the drawback on the writing property. Examples of the improvements are: inclusion of an inorganic pigment having a size of 1-40 .mu.m in the back side polyolefin resin layer (Japanese Patent Provisional Publication No. 55(1980)-43528); provision of a layer comprising an aqueous silica sol and a water-soluble polymer such as polyvinyl alcohol or carboxymethylcellulose (Japanese Patent Publication No. 44(1969)-14884); provision of a layer comprising a aqueous silica sol and a water-insoluble polymer emulsion such as polyethylene emulsion (Japanese Patent Publication 50(1975)-36565); and provision of a coating layer containing a pigment such as clay (Japanese Utility Model Provisional Publication No. 52(1977)-169426).
However, these prior arts have their own drawbacks. For instance, the inclusion of an inorganic pigment having a size of 1-40 .mu.m in the back side polyolefin resin layer is considered to be difficultly employed in practice, because the resin layer deteriorates by the inclusion, resulting in occurrance of cracks, etc., and pollution with the pigment is introduced into the preparation stage. In the case of providing a coating layer having the known composition, since the coating amount has to be large, such as around 5 g/m.sup.2 and in certain cases more than 10 g/m.sup.2, a number of restrictions are introduced into the process, for instance, in regard of the drying stage. Also diadvantageous is that the coating layer is liable to break away or dissolve in the photographic development process, or that the pigment is liable to break away from the coating layer under contact with other face after the developmemt stage.